Optical communication technology relies on wavelength division multiplexing (WDM) to provide increased bandwidth over existing installed fiber optic lines, as well as newly deployed fiber optic line installations. Several technologies exist to provide the technical solution to WDM: array waveguide gratings (AWG's), fiber Bragg grating based systems, interference filter based systems, Mach-Zehnder interferometric based systems, and diffraction grating based systems, to name a few. Each system has advantages and disadvantages over the others.
Diffraction grating based systems have the advantage of parallelism, which yields higher performance and lower cost for high channel count systems. In particular, a diffraction grating is a device that diffracts light by an amount varying according to its wavelength. For example, if sunlight falls on a diffraction grating at the correct angle, the sunlight is broken up into its individual component colors (i.e., rainbow).
Gratings work in both transmission (where light passes through a material with a grating written on its surface) and in reflection (where light is reflected from a material with a grating written on its surface). In optical communications, reflective gratings have a widespread use. A reflective diffraction grating includes a very closely spaced set of parallel lines or grooves made in a mirror surface of a solid material. A grating can be formed in most materials wherein the optical properties thereof are varied in a regular way, having a period that is relatively close to the wavelength. Incident light rays are reflected from different lines or grooves in the grating. Interference effects prevent reflections that are not in-phase with each other from propagating.
There are two primary groove profiles in conventional diffraction gratings, blazed gratings and sinusoidal gratings. The blazed grating includes a jagged or sawtooth shaped profile. The sinusoidal grating has a sinusoidal profile along the surface of the grating.
The diffraction equation for a grating is generally described byGmλ=n(sin(α)+sin(β)) where, G=1/d is the groove frequency in grooves per millimeter and d is the distance between adjacent grooves, m is the diffraction order, λ is the wavelength of light in millimeters, α is the incident angle with respect to the grating normal, β is the exiting angle with respect to the grating normal, and n is the refractive index of the medium above the grooves.
FIG. 12A is a representative pictorial showing optical characteristics of a blazed diffraction grating in reflecting a narrowband optical signal. The blaze diffraction grating 900 is defined by certain physical parameters that effect optical performance. These physical parameters include the reflection surface material, the number of grooves g per millimeter, blaze angle θB, and the index of refraction of an immersed grating medium 902. The reflection surface 905 typically resides on a substrate 910.
As shown on FIG. 12A, the groove spacing is defined by d. An incident narrowband optical signal with a center wavelength λ1 has an incident angle α1 (measured from the grating normal Nq) and a reflection angle β1 (also measured from the grating normal Ng). The angle between the grating normal Ng and the facet normal Nf defines the blaze angle θB.
As previously discussed, when narrowband light is incident on a grating surface, it is diffracted in discrete directions. The light diffracted from each groove of the grating combines to form a diffracted wavefront. There exists a unique set of discrete or distinct angles based upon a given spacing between grooves that the diffracted light from each facet is in phase with the diffracted light from any other facet. At these discrete angles, the in-phase diffracted light combine constructively to form the reflected narrowband light signal.
A sinusoidal diffraction grating is similarly described by the equation above. When α=β, the reflected light is diffracted directly back toward the direction from which the incident light was received. This is known as the Littrow condition. At the Littrow condition, the diffraction grating equation becomesm*λ=2*d*n*sin (α), where n is the index of refraction of the immersed grating medium 902 in which the diffraction grating is immersed.
FIG. 12B is a representative pictorial showing optical characteristics of a sinusoidal diffraction grating. Sinusoidal gratings, however, do not have a blaze angle parameter, but rather have groove depth (d). An immersed grating medium 955 resides on the sinusoidal grating 950 having a certain index of refraction, n. The diffraction grating equation discussed above describes the optical characteristics of the sinusoidal diffraction grating 19d based upon the physical characteristics thereof.
FIG. 12c shows a polychromatic light ray being diffracted from a blazed grating 960. An incident ray (at an incident angle θ1 to the normal) is projected onto the blazed grating 960. A number of reflected and refracted rays are produced corresponding to different diffraction orders (values of m−0, 1, 2, 3 . . . ). The reflected rays corresponding to the diffraction order having the highest efficiency (i.e., lowest loss) are utilized in optical systems.
As with most communications systems, there is a need to provide improved optical transmission rate and more efficient propagation of the communication signals in the fiber optic communication system. By improving the efficiency and/or decreasing the loss of the communication signals, the need to install optical repeaters and/or optical amplifiers is reduced, thereby decreasing operating costs of the system. Furthermore, an increase in signal efficiency reduces demand on fiber optic lines in a system, thereby reducing the need for burying additional optic lines. The burying of additional fiber optic cable is quite costly as it is presently on the order of $15,000 to $40,000 per kilometer.
Because WDM devices generate optical signals, one area of improvement is focused on the insensitivity to signal polarization. As is well known, the polarization of a signal affects the speed at which pulse energy in the signal's polarization modes or states propagate in an optical fiber. As a result, polarized signals generally cause significant timing and signal reconstruction problems within an optical system.
Ultimately, signal performance within a WDM device is attributable to a great extent to the performance of the diffraction grating therein. Because the parameter values which describe the diffraction grating often dictate the efficiency and the polarization effects of diffracted optical signals, much time, money, and effort have been dedicated to determining diffraction grating parameter values to effectuate improved transmission performance. Due in part to the number of diffraction grating parameters, the considerable range of corresponding parameter values, and the interdependencies between the diffraction grating parameters, designing and implementing a diffraction grating yielding improved performance are nontrivial.
In this regard, designing diffraction gratings must additionally take into account real-world effects that can only be measured empirically to determine if the theoretical parameters for a diffraction grating yield a viable solution. For example, one difficulty in creating improved diffraction gratings is the prolonged time period for creating a master diffraction grating. A single diffraction grating master may take several weeks to produce. Although the master diffraction grating, having a specific set of grating parameters, may yield acceptable results (i.e., low loss or a partially polarization insensitive result), a replicated diffraction grating created from the master diffraction grating may produce less than desirable signal performance characteristics. Consequently, the process of designing and developing diffraction gratings (determining grating parameters that yield good signal and/or master grating related characteristics, producing a master diffraction grating having the determined grating parameters and producing a replicated diffraction grating from the master diffraction grating that yields good signal performance characteristics) so as to produce a diffraction grating having improved performance requires solving both theoretical and practical problems.
Based upon the foregoing, there is a need for a diffraction grating for employment within an optical system having improved signal performance.